This invention relates to a framing tube that optically detects physical phenomenon changing at very high speed and a camera incorporating such a framing tube.
For instance, continuous images of a nuclear fusion phenomenon changing at very high speed, which occurs when a capsule of heavy hydrogen is explosively condensed with the laser beam, reproduced with high accuracy in terms of time can be a useful piece of data for the development of nuclear fusion reactors. This type of image reproducing calls for a very short exposure time and interval. The framing camera is a device used for such purpose, and the framing tube is a vacuum tube that constitutes the main part of the framing camera. Conventional framing cameras and tubes are complex in structure and operation, as described below. Besides, they cannot insure exact exposure time and interval.
FIG. 1 shows an example of conventional framing cameras. Reference numeral 1 designates a framing tube in cross section, which comprises a cylindrical airtight vacuum container 11 in which a photocathode 13 is provided on the inside of a first transparent end 12 thereof and a fine-mesh electrode 14 is disposed parallel and close to said photocathode. A fluorescent screen 18 is provided on the inside of a second transparent end 19 of the cylindrical airtight container 11, with deflecting electrodes 16 and 17 disposed, one above the other, in such a manner as to allow the passage of photoelectron beams from the fine-mesh electrode 14 to the fluorescent screen 18 therebetween. Reference numeral 15 designates a focusing electrode. The electric field due to the focusing electrode 15 focuses photoelectron beams from the photocathode 13, on the fluorescent screen 18 to form the optical image thereon, corresponding to the electronic image on the photocathode 13. A direct current power supply 24 holds the photocathode at a potential lower than that of the fluorescent screen 18. A direct current power supply 23 and a resistor 26 keep the fine-mesh electrode at a still lower potential. Accordingly, even when an optical image A is projected on the photocathode 13, its photoelectrons are cut off by the fine-mesh electrode 14. The photoelectrons pass through the fine-mesh electrode 14 only at a moment when a pulse power supply 20 applies a positive rectangular pulse as shown at I in FIG. 2 to the fine-mesh electrode 14 through a capacitor 25. During the time in which several such pulses are produced, a ramp generator 27 applies a ramp voltage that sweeps photoelectron beams from one end of the fluorescent screen 18 to the other end thereof, as shown at II in FIG. 2, to between the deflecting electrodes 16 and 17. Consequently, the optical image A is reproduced on the fluorescent screen 18 as a plurality of images A.sub.1, A.sub.2, A.sub.3, and so on, varying at intervals at which the pulses are produced. Because of the technical difficulty in pulse generation, however, such a framing camera cannot provide an exposure time shorter than 10 nanoseconds. The exposure interval depends on the time required for deflecting the photoelectron beams so that the second image A.sub.2 should not overlap the first image A.sub.1. Namely, the exposure interval is determined by the rate at which the voltage supplied from the ramp generator 27 changes, the limit being 50 nanoseconds. A clearer image will be obtained if a stepped voltage which is constant when the pulse power supply 20 sends forth the rectangular pulse and which changes when it stops the pulse supply, as shown at III in FIG. 2, is applied between the deflecting electrodes 16 and 17 in place of the ramp voltage. A negative pulse may be applied on the photocathode 13 instead of applying the positive pulse on the fine-mesh electrode 14, but the limit on the exposure time and interval remains unchanged.
FIG. 3 shows another example of conventional framing cameras. Reference numeral 3 designates a framing tube in cross section, which comprises a cylindrical airtight vacuum container 31 in which a photocathode 33 is provided on the inside of a first transparent end 32 thereof and a fluorescent screen 40 on the inside of a second transparent end 41 thereof. Between the photocathode 33 and fluorescent screen 40 is provided a slit plate 37 which is parallel thereto, the slit plate 37 having a plurality of parallel slits 371 372 and 373. Between the photocathode 33 and slit plate 37 are disposed paired deflecting electrodes 35 and 36, one above the other, in such a manner as to allow the passage of photoelectrons therebetween. Deflecting electrodes 38 and 39 are disposed between the slit plate 37 and fluorescent screen 40 in a similar fashion. Element 34 is a focusing electrode. The electric field due to the focusing electrode 34 focuses photoelectron beams from the photocathode 33 on the fluorescent screen 40 to form the optical image thereon, corresponding to the electronic image on the photocathode 33. Since a direct current power supply 42 keeps the photocathode 33 at a potential lower than that of the slit plate 37, the photoelectrons released when an optical image B is projected on the photocathode 33 strike against the slit plate 37. If a ramp generator 48 then applies a ramp voltage across the deflecting electrodes 35 and 36, the electron beams are swept at right angles with the slits 371, 372 and 373, whereupon an image formed by the photoelectron beams passes, successively from one end thereof, through the slits 371, 372 and 373 at intervals that depend on the sweeping rate and slit intervals. The photoelectron beams having passed through the slits 371, 372 and 373 make up a plurality of optical images B changing between the time intervals dependent upon the sweeping rate and slit intervals and are arranged in a line in the order in which time passes. When the ramp generator 47 applies ramp voltages of opposite polarities to the deflecting electrodes 38 and 39, the time-lagged optical image B are reproduced on the fluorescent screen as a plurality of images B.sub.1, B.sub.2 and B.sub.3. The image B.sub.1 , B.sub.2 and B.sub.3 obtained by this framing camera are not the reproduction of the different parts of the optical image B at one time, but those at different times. With a large portion of the photoelectron beams cut off by the slit plate 37, only a small portion thereof is utilized in reproducing the image on the fluorescent screen 40. With this type of framing camera, the exposure time and intervals are determined by the limit of speed with which the voltage produced by the ramp generators 47 and 48 changes. It is therefore difficult to obtain the exposure time which is not longer than the 100 picoseconds which is necessary for the electron beam image to cross the slit and the exposure interval of not longer than 50 nanoseconds which is necessary for the images B.sub.1 and B.sub.2 not to overlap each other on the fluorescent screen 40.
FIG. 4 shows a third example of conventional framing cameras. Reference numeral 5 designates a framing tube in cross section, which comprises a cylindrical airtight vacuum container 51 in which a photocathode 53 is provided on the inside of a first transparent end 52 thereof and a fluorescent screen 63 on the inside of a second transparent end 64 thereof. Between the photocathode 53 and fluorescent screen 63 are provided shutter electrodes 56 and 57, correcting electrodes 59 and 60, and shifting electrodes 61 and 62. A direct current power supply 68 is connected to the photocathode 53 and fluorescent screen 63 to keep the photocathode 53 at a potential lower than that of the fluorescent screen 63. When an optical image C is projected on the photocathode, a repetitive deflecting voltage generator 65 applies a wavy repetitive deflecting voltage as shown at IV in FIG. 5 to the shutter electrodes 56 and 57, thereby deflecting the electron beams from below to above. When a deflecting voltage having the same waveform as and opposite in phase to or slightly lagging behind the repetitive deflecting voltage applied to the shutter electrodes 56 and 57 is applied to the correcting electrodes 59 and 60, the electron beams pass through the space between the correcting electrodes 59 and 60 at certain moments of time. When a ramp generator 66 applies a ramp voltage as shown at V in FIG. 5, to the shifting electrodes 61 and 62 which thereby causes the electron beams to sweep across the fluorescent screen 63, the optical image C is reproduced on the fluorescent screen 63 as a plurality of images C.sub.1, C.sub.2 and C.sub.3 that vary at intervals each of which equals 1/2 of the cycle in which the waveform of the repetitive voltage changes. In FIG. 4, reference numeral 54 denotes a focusing electrode, 55 an anode electrode, and 58 an aperture plate. The electric field due to the focusing electrode 54 focuses photoelectron beams from the photocathode 53 on the fluorescent screen 63 to form an optical image thereon corresponding to the electronic image on the photocathode 53. This kind of framing camera requires a repetitive deflecting voltage generator, in addition to the ramp generator. The exposure time, which depends upon the rate of change of the repetitive deflecting voltage, cannot be made shorter than 10 nanoseconds and the exposure intervals, which depends upon the cycle of the repetitive deflecting voltage, cannot be made shorter than 50 nanoseconds. In this example too, a clearer image will be obtained if a stepped voltage that is constant when the repetitive deflecting voltage is approximately 0 volts and changes at other times, as shown at VI in FIG. 5, is applied to the shifting electrodes 61 and 62 instead of the ramp voltage.
The deflecting voltage used for the three types of framing camera described above must change over an amplitude of more than several kilovolts and at a speed of approximately 1 volt per picoseconds. But it is technically very difficult to generate such a voltage that changes over such a wide amplitude and with such a high speed.